Traditional batteries or supercapacitors are typically two-dimensional (2-D) cells, where thick films of the anode, separator/electrolyte, and cathode are stacked, spiral wound, or folded. The electrodes (anodes and cathodes) of supercapacitors, Li metal or Li-ion cells are of similar form and are made by similar processes on similar or identical equipment where active electrode materials in the form of powder are mixed with conductive binder to form a slurry, which is coated on metallic foils that act as the current collectors conducting the current in and out of the cell. The electrodes are dried, slit, and assembled into 2-D cells. To maximize power density, films of electrode, electrolyte and current collector are kept as thin as possible to achieve smallest distance for ion transport. However, these electrode materials have a particle size normally larger than 10 microns and it is not practical to prepare electrode films thinner than several tens of microns. In addition, to maintain sufficient electrochemical stability and mechanical integrity necessary for cell fabrication, the thickness of inactive component including current collector and electrolyte separator is typically limited to 25 microns. For balanced power and energy performance, electrode thickness is typically set to 100 microns. There is therefore no additional room for improving electrochemical performance characteristics by reducing film thickness of electrodes, electrolytes and current collectors.
In recent years there has been the realization that unproved battery performance can be achieved by reconfiguring the electrode materials that currently employed in 2-D batteries into 3-D architectures. The 3-D configurations generally include large number of micron- or nano-sized cathodes and anodes separated with or distributed in the matrix of a solid electrolyte domain. One example of the 3-D microbatteries was illustrated in a review article (Long et al, Chem. Rev. 2004, 104, 4463-4492), where arrays or rows of interdigitated cylindrical cathodes and anodes are attached at their bases to flat sheets serving as current collectors. Another example of 3-D microbattery was illustrated in a US patent (U.S. Pat. No. 8,795,885 B2) where an anode including an array of nanowires electrochemically coated with a polymer electrolyte, and surrounded by a cathode matrix. The general features of the 3-D configuration ensure short ion transport distances within and between cathodes and anodes and may permit use of smaller amounts of inactive materials including current collectors and separators. Consequently, energy storage devices with appropriate 3-D structures possess performance characteristics of high power density and high energy density along with other desirable characteristics including long cycle and shelf life and excellent safety that are not seen in traditional 2-D configurations.
However, the development of 3-D technologies for energy storage devices is inherently limited by the complexity of micro fabrications involving varieties of special battery and supercapacitor materials. It is therefore extremely challenging to fabricate 3-D energy devices and no operational 3-D energy devices have been reported. In addition, these micro fabrication techniques, developed for fabrication of micro-sized devices, are not best suited to fabrication of regular sized devices including batteries and supercapacitors for mobile and portable electronic applications.